This invention relates to crystalline galliosilicates and is particularly concerned with a crystalline galliosilicate molecular sieve having the zeolite omega structure, methods of producing such a molecular sieve, catalysts containing such a molecular sieve and processes for using catalysts containing such a molecular sieve.
Zeolites are well known natural and synthetic molecular sieves that can be defined as crystalline, three-dimensional aluminosilicates consisting essentially of alumina and silica tetrahedra which interlock to form discrete polyhedra. The polyhedra are interconnected to form a framework which encloses cavities or voids interconnected by channels or pores. The size of the cavities and pores will vary depending on the framework structure of the particular zeolite. Normally, the cavities are large enough to accommodate water molecules and large cations which have considerable freedom of movement, thereby permitting sorption, reversible dehydration and ion exchange. The dimensions of the cavities and pores in a zeolite are limited to a small number of values and can vary from structure to structure. Thus, a particular zeolite is capable of sorbing molecules of certain dimensions while rejecting those of dimensions larger than the pore size associated with the zeolite structure. Because of this property zeolites are commonly used as molecular sieves.
In addition to their molecular sieving properties, zeolites show a pronounced selectivity toward polar molecules and molecules with high quadrupole moments. This is due to the ionic nature of the crystals which gives rise to a high nonuniform electric field within the micropores of the zeolite. Molecules which can interact energetically with this field, such as polar or quadrupolar molecules, are therefore sorbed more strongly than nonpolar molecules. This selectivity toward polar molecules is the unique property of zeolites which allows them to be used as drying agents and selective sorbents.
The pore size of a zeolite can vary from about 2.6 Angstroms for analcime to about 10.0 Angstroms for zeolite omega. The term "pore size" as used herein refers to the diameter of the largest molecule that can be sorbed by the particular zeolite or other molecular sieve in question. The measurement of such diameters and pore sizes is discussed more fully in Chapter 8 of the book entitled "Zeolite Molecular Sieves," written by D. W. Breck and published by John Wiley & Sons in 1974, the disclosure of which book is hereby incorporated by reference in its entirety. The pore size range of 2.6 to 10.0 Angstroms is particularly suited for molecular separation and catalytic processing. Analcime will sorb ammonia while excluding larger molecules whereas zeolite omega will sorb perfluorotributyl amine [(C.sub.4 F.sub.9).sub.3 N] while excluding any molecule having a diameter greater than about 10.0 Angstroms. All of the other approximately 150 zeolites now known have pore sizes falling within the range between 2.6 and 10.0 Angstroms.
In addition to their use as drying agents and selective sorbents, zeolites are widely used as components of chemical conversion catalysts. As found in nature or as synthesized, zeolites are typically inactive because they lack acid sites. In general, acid sites are created by subjecting the zeolite to an ion exchange with ammonium ions followed by some type of thermal treatment which creates acid sites by decomposing the ammonium ions into gaseous ammonia and protons. Activated zeolites have been used in many types of chemical conversion processes with the smaller pore zeolites being used to selectively sorb and crack normal and moderately branched chain paraffins.
Because of the unique properties of zeolitic molecular sieves, there have been many attempts at synthesizing new molecular sieves by either substituting an element for the aluminum or silicon present in zeolitic molecular sieves or adding another element in addition to the aluminum and silicon. The term "zeolite" or "zeolitic" as used herein refers to molecular sieves whose frameworks are formed of substantially only silicon and aluminum atoms in tetrahedral coordination with oxygen atoms. One such class of new molecular sieves that has been created is that in which all the framework aluminum has been replaced by gallium. Specifically, it has been reported in the literature that galliosilicate molecular sieves having the faujasite structure, the pentasil structure and the mordenite structure have been synthesized. The synthesis of a galliosilicate analogue of Theta-1 zeolite has also been reported. It is believed, however, that there has been no reported instance of a substantially alumina-free galliosilicate with the zeolite omega structure having been synthesized.
Accordingly, it is one of the objects of the present invention to provide a crystalline, galliosilicate molecular sieve free of alumina with the zeolite omega structure and a method for preparing such a molecular sieve, which sieve may be useful in many types of chemical conversion processes, particularly hydrocarbon conversion processes. This and other objects of the invention will become more apparent in view of the following description of the invention.